Regardless of the type of internal combustion engine (for example reciprocating piston engine, pistonless rotary engine or free-piston engine), noise is generated as a result of the successively executed strokes (in particular intake and compression of the fuel-air mixture, combustion and discharge of the combusted fuel-air mixture). The noise propagates through the internal combustion engine in the form of solid-borne sound and is emitted on the outside of the internal combustion engine in the form of airborne sound. The noise also travels in the form of airborne sound together with the combusted fuel-air mixture through an exhaust system that is in fluid communication with the internal combustion engine. Due to this, also flow noise adds to the noise from the internal combustion engine. The noise traveling through the exhaust system is referred to as exhaust noise.
This noise is often regarded as being disadvantageous. There are statutory provisions on protection against noise to be observed by manufacturers of vehicles driven by internal combustion engines. These statutory provisions normally specify a maximum sound pressure for an operation of a vehicle. Further, manufacturers also try to impart a characteristic noise emission to internal combustion engine driven vehicles of their production, with the noise emission fitting the image of the respective manufacturer and being popular with customers. Present-day engines with small displacement often cannot naturally generate such intended characteristic noise.
The noise propagating through the internal combustion engine in the form of solid-borne sound can be muffled quite well and is thus usually no problem as far as protection against noise is concerned.
The noise traveling through the exhaust system of the internal combustion engine together with the combusted fuel-air mixture in the form of airborne sound is reduced by exhaust mufflers located ahead of the exhaust system discharge port and downstream of catalytic converters, if present. Respective mufflers may, for instance, work according to the absorption and/or reflection principle. Among others, resonance absorbers are used that operate according to the Helmholtz resonator principle.
A Helmholtz resonator consists of a body enclosing an air volume, the body comprising a resonator neck having an opening connecting the air volume with the surroundings. Due to the opening in the resonator neck, the air volume is not surrounded by the body completely, but can be considered divided into first and second sub-volumes of air. The first sub-volume of air is defined by the geometry of the resonator neck and extends from the opening in the resonator neck along the entire length of the resonator neck. The size of the first sub-volume of air thus depends on the cross section and the length of the resonator neck. The resonator neck's cross-section may either vary along the length of the resonator neck or remain constant. The resonator neck may further be straight-lined or curved. The second sub-volume of air adjoins the first sub-volume of air inside the body directly, the resonator neck thereby separating it from the body's opening. The second sub-volume of air being bigger than the first sub-volume of air is defined by the body's geometry exclusive of the resonator neck. At the transition between the first and the second sub-volumes of air, there may either be an opening in the body wall or the sign of the body wall's curvature will change, for example. The elasticity of the air volume inside the body combines with the inertial mass of the air present in the resonator neck to form a mechanical mass-spring system. Subject to the shape of the air volume, the mass-spring system has either one (for a spherical shape) resonance frequency (natural frequency) or a plurality (for shapes different to a sphere) of resonance frequencies (natural frequencies). The natural frequency depends inter alia on the size of the air volume enclosed, the cross-sectional area of the opening in the resonator neck, the length of the resonator neck, and a port adjustment factor depending on the ports shape and configuration (e.g. round, angular shaped, slit-like).
Both operating modes (absorption and/or reflection principle) suffer from lacking any adaptation to the frequency spectrum of a noise traveling through an exhaust system and changing with a changing speed of an internal combustion engine. An optimum noise muffling with conventional mufflers is therefore rarely achieved. The flow resistance for the exhaust traveling through the exhaust system presents a problem common to both operating modes. The noise muffling often is insufficient when designing the mufflers for the maximum exhaust gas stream at high speeds of the internal combustion engine. When designing the mufflers for an average exhaust gas stream at medium speeds of the internal combustion engine, a significant increase in the flow resistance and thus in the consumption of the internal combustion engine at higher engine speeds will be the consequence.
A muffler is known from the European patent EP 1 760 279 B1 wherein a switchable muffler pipe is acoustically coupled to a muffling system such that the muffling system is active, i.e. has its respective muffling effect, both for the open and the closed pipe but has a different muffling characteristic for the open pipe than for the closed pipe. With this design, the switchable pipe forms part of an effective muffling system also in its closed state, whereby the pipe changes its muffling characteristic when opened but still remains active.
Mufflers operating according to the Helmholtz resonator principle are known from U.S. Pat. No. 3,613,830, U.S. Pat. No. 4,501,341 and U.S. Pat. No. 5,602,368. Further mufflers are known from U.S. Pat. No. 2,112,964 and DE 10 2008 062 014 A1.